This invention relates to enzyme inhibitory substances. In particular, it relates to substances which inhibit the action of angiotensin-converting enzyme, referred to hereinafter as ACE, and to a method for the manufacture thereof. The ACE inhibitory factors provided by this invention are arbitrarily designated A-58365 factors A, B, and C and are produced by culturing a new strain of Streptomyces chromofuscus, NRRL 15098. This invention further provides a method for the treatment of hypertension with an A-58365 factor.